María Natalia Umaña and Inés Ibañez, University of Michigan, discuss their article: How do drought and elevated temperatures influence CO2 fertilization effects on tree seedling performance? A global meta-analysis
Climate change is not happening one thing at a time. In forests across the globe, seedlings are already experiencing a mix of higher atmospheric CO₂, rising temperatures, and more intense droughts. Yet most experiments still test these factors one by one. What happens when all of them hit at once?
To find out, we conducted a meta-analysis of over 80 experiments on tree seedlings. We focused on the early life stages, germination, establishment, and seedling growth, because they are particularly vulnerable to environmental change and essential for forest regeneration. Our goal was to understand the interaction between different drivers of climate change (elevated CO₂, rising temperatures, and intense droughts), including whether their combined effects are simply additive (sum of individual impacts), or if they amplify (synergistic) or cancel each other out (antagonistic).
Why focus on seedlings?
Seedlings are not just small versions of adult trees. They face different physiological constraints and have shallower roots, making them especially sensitive to heat and water stress. In disturbed forests where seedlings often face harsher microclimates, their vulnerability is even more pronounced. If seedlings cannot survive and grow, forests cannot regenerate, and the ecological and carbon-storage functions they provide will be lost.
What we found
The most consistent result was additivity, in most cases, the combined effects of CO₂, drought, and warming added up without intensifying or buffering each other. There were few surprises, but also few buffers: the negative effects of drought were not offset by CO₂, and warming did not make things significantly worse unless water was also limited.
In fact, drought consistently emerged as the most limiting factor. Elevated CO₂ alone often enhanced growth, and in some cases, those gains persisted with warming. But once drought entered the picture, those benefits disappeared. Even doubling CO₂ levels did not fully compensate for water stress.
There were also some antagonistic effects, particularly in germination and photosynthesis, where the combined impact of drought and warming was less severe than expected. These outcomes may reflect temporary physiological buffers, like improved enzymatic activity under warmth, or CO₂-driven water-use efficiency, but they are unlikely to last under more extreme drought.
A need for diversity in data and species
One limitation that our meta-analysis revealed was the lack of tropical species in these types of experiments. Most experiments have been conducted in temperate and boreal systems, raising questions about how generalizable current findings are. We also saw hints of genus-specific responses, for example, Pinus and Quercus seedlings were more sensitive to drought and warming, while Eucalyptus sometimes benefited from elevated CO₂ and temperature.
Interestingly, plant responses were consistent across traits, what seedlings did at the leaf level (like photosynthesis) aligned with whole-plant outcomes (like biomass). This suggests coordinated strategies, even under stressful conditions.
What does this mean for forests?
Our results show that CO₂ alone won’t save seedlings from the harsher conditions of the future. While there are some benefits under elevated CO₂ and moderate warming, drought clearly overrides them. Forest regeneration, especially in dry years, may be more vulnerable than previously thought.
We hope this study encourages broader, more coordinated experiments that explore how multiple climate factors interact, and that it pushes us to expand research to more diverse ecosystems and taxa. After all, the seedlings we study today are the forests of tomorrow.
